As a printing apparatus for printing images from a personal computer, there have been a printer employing three types of ink each being different in terms of color such as cyan (C), magenta (M) and yellow (Y), and a printer employing four types colors wherein black (K) ink is added to the aforesaid three types of ink. In these printers, images are formed through on-off operations of each dot having its hue obtained by half-toning.
In recent years, on the other hand, for obtaining output images having higher image quality, there is observed the spread of a printer wherein dots with plural gradation levels for a single hue are formed for printing, by using ink having different concentrations (for example, dark color ink and light color ink) in the same hue or by making a dot diameter of ink to be different each other.
However, even when dots having plural gradation levels with a single hue can be formed as stated above, there is sometimes an occasion where an image is rather worsened if the dots are not arranged properly. For example, for obtaining output images having dark dots (dots having higher concentration per unit area) and light dots (dots having lower concentration per unit area), when halftone processing of binarization is conducted for each dot, and when respective pixel values are summed up afterwards, and those where both are superposed do not show excellent dispersion totally, although dot dispersion for light dots only (see FIG. 19(a) and dot dispersion for dark dots only (see FIG. 19(b) are in the excellent condition, thus, final graininess is worsened considerably (see FIG. 19(c).
Therefore, there have been proposed various methods concerning a method of arrangement of plural types of dots each having a different concentration per unit area (for example, see Patent Documents 1-4). Among them, in Patent Document 1, 100% or more of light dots are dotted, and when the difference of concentration from dark dots becomes small, the dark dots are started to be put in.
In Patent Document 2, an occupancy rate of large dots is calculated from input values to arrange dots and then, 3 level halftone processing of input values is carried out, and any of large dot, small dot and no dot is allotted.
Further, in Patent Document 3, input values are dissolved into the dot rate between dark dots and light dots, and for improving dispersion of dark dots, halftone processing is carried out first for dark dot, and then, halftone processing is carried out with a total value of dark dot and light dot, so that arrangement of dark dots may be determined.
Further, in Patent Document 4, input values are dissolved into the dot occupancy rate between dark dots and light dots, and dark dots are selected from the dot occupancy rate of dark dots by the use of blue noise dither, while, light dots are selected from the total value of the dot occupancy rate between dark dots and light dots further by the use of blue noise dither.
(Patent Document 1) Unexamined Japanese Patent Application Publication No. 2003-80682
(Patent Document 2) Japanese Patent Publication No. 3580150
(Patent Document 3) Japanese Patent Publication No. 3208777
(Patent Document 4) Japanese Patent Publication No. 3292104
However, the aforesaid Patent Document 1 has a problem that an amount of ink cannot be limited, and types of usable sheets are limited.
The Patent Document 2 has a problem that excellent images are not obtained in respective dot dispersions, especially in dispersion of remarkable large dots, and graininess is worsened accordingly.
In the Patent Document 3, granulation feeling is rather increased especially when weight of a dot is considered, because dispersion of light dots is disturbed by dark dots, which is a problem.
In the Patent Document 4, under the condition where the sum of the recording rates for two types of dots exceeds 100%, a dither matrix wherein sizes of threshold values of dither matrix used for judgment of forming of dots of a type on one side are replaced is compared with the recording rate of the type on the other side, thus, on the portion where two types of dots are superposed each other, the two types of dots are super posed on a random basis without being controlled, resulting in deterioration of graininess, which is a problem.
An objective of the invention therefore is to provide an image processing apparatus and an image processing method wherein dot dispersion can be carried out excellently when forming the images on a recording medium by the use of dots in a plurality of types.